(function (doc, win) {
               var docEl = doc.documentElement,
                       resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
                       recalc = function () {
                            var clientWidth = docEl.clientWidth;
                            if (!clientWidth) return;
                            if(clientWidth>=640){
                                 docEl.style.fontSize = '100px';
                            }else{
                                 docEl.style.fontSize = 100 * (clientWidth / 750) + 'px';
                            }
                       };

               if (!doc.addEventListener) return;
               win.addEventListener(resizeEvt, recalc, false);
               doc.addEventListener('DOMContentLoaded', recalc, false);
          })(document, window);